Oh-Kwan Kwon

Tribological behaviour of plasma-sprayed chromium oxide coating

Abstract A plasma-sprayed chromium oxide coating was studied to gain a better understanding of its tribological behaviour. Both dry and lubricated wear tests were conducted with a reciprocating motion at room temperature and at 450°C for the dry wear tests, and at room temperature and 200°C for the lubricated wear tests. Under dry sliding conditions, dispersed smooth surface films were formed by plastic deformation of compacted debris particles that adhered to the surface and these films strongly influence the friction coefficient and wear rate of the coating. Considerable quantity of CrO3 was detected at room temperature whereas CrO2 was detected at 450°C. CrO2 in the smooth film seems more favorable than CrO3 in reducing friction. Under lubricated sliding conditions, tribochemical reaction films of different species of carbon–oxygen bond units were formed depending on the test temperature. The surface films appear to be effective in reducing friction and preventing wear. The dominant species with carbon-associated bonds in the film was identified as graphite at both room temperature and high temperature. Among the compounds formed in the film, graphite appears to be most important in determining the tribological performance of the film.

Classification of wear debris using a neural network

Abstract In this work, an evaluation was made to prove the possibility of employing a neural net method for the classification of debris and monitoring of a lubricated contact pair. We trained a neural net to classify the severity of wear into two types from the morphological features of the wear debris. The following procedures were used. First, the shape of wear particles was characterized by Fourier descriptors. The Fourier descriptors were considered as coordinates of a point in multidimensional feature space. A set of points from a cluster, and the location and structure of the cluster depend on the morphology of the wear particles and the current conditions of the contact system. A distance distribution between the debris in the feature space was used to represent the location of the cluster. Second, we trained a back-propagation neural net. To train the neural net, we used the distance distribution corresponding to the different stages of the wear process as an input vector and encoded the wear rate as a desired response. The network was then further trained until the desired error goal was achieved. Finally, we tested the trained neural net. The ability of the neural net method to monitor wear is shown.

Evaluation of frictional characteristics for a pin-on-disk apparatus with different dynamic parameters

Frictional characteristics in response to changes of the dynamic parameters of a pin-on-disk apparatus were experimentally and theoretically investigated for dry sliding. The apparatus has been designed and built to allow control of sliding speed, load, stiffness and loading mechanism. Dynamic friction and normal force were measured for various conditions of the system parameters, e.g. different stiffness in the normal direction, loading mechanisms and test materials. For a normal load, mechanisms using a dead weight, a compression-type spring and a pneumatic cylinder were selectively applied. Stiffness in the loading direction was also varied using a spring with different stiffness. Steel, rosin and polytetrafluoroethylene were tested because they have different types of frictional behavior. Test results showed that frictional characteristics at various loading mechanisms were different even though the operating parameters were the same. They are also strongly dependent on both the sliding velocity and the run-out of the disk surface. Effects of the surface run-out on the friction behavior were further evaluated using a tapered steel disk. When the surface run-out was high, the coefficient of friction was computed differently with the data processing. A theoretical analysis for a simple model of a pin-on-disk apparatus showed that surface irregularities, such as surface roughness, waviness and run-out, result in dynamic normal forces which depend on both the dynamic parameters of the system and the sliding velocity.

Wear behaviour of plasma-sprayed partially stabilized zirconia on a steel substrate

Abstract In this experimental investigation, various layers of partially stabilized zirconia (PSZ) coated on steel by plasma spraying were studied to gain a better understanding of the tribological behaviour of PSZ under the conditions of dry contact in a reciprocating motion at a temperature of 200 °C, which represents the severe operating conditions of friction pairs, such as cylinder liner-piston ring. The parameters investigated in relation to the wear behaviour were the thickness of the coating, oscillating frequency, sliding cycle and applied load. To identify the wear mechanism, X-ray diffraction, scanning electron microscopy and optical microscopy were used. In comparison with a steel-steel pair, PSZ coating-steel and PSZ coating-PSZ coating pairs exhibit far better wear characteristics and, therefore, the distinct possibility of applying a plasma-sprayed PSZ coating as a cylinder liner has been confirmed. The oscillating frequency and applied load conversely affect the wear rate of disc. The recommended coating thickness was found to be in the range 180–300 μm to ensure good anti-wear behaviour without the expense of high coating cost.

ELECTRORHEOLOGICAL BEHAVIOR OF CHITOSAN DERIVATIVE SUSPENSIONS

The electrorheological (ER) behavior of chitosan and chitosan phosphate suspensions in silicone oil was investigated. Chitosan and chitosan phosphate suspensions showed a typical ER response (Bingham flow behavior) upon application of an electric field. However, chitosan phosphate suspension exhibited excellent shear yield stress compared with chitosan suspension. The difference in behavior results from the difference in the conductivity of the chitosan and chitosan phosphate particles due to their degree of the polarizability. The shear stress for chitosan and chitosan phosphate suspensions showed a linear dependence on the volume fraction of particles. The values of structure factor, As obtained 1 and 3~4 for chitosan and chitosan phosphate suspensions and it may be due to the formation of single-row chains and multiple chains upon application of the electric field. Throughtout the experimental results, chitosan and chitosan phosphate suspensions were shown to be an ER fluid.

Electro-rheological properties of anhydrous ER suspensions based on phosphoric ester cellulose particles

The electro-rheological (ER) behaviour of suspensions in silicone oil of phosphoric ester cellulose particles (average particle size 17.77 µm) was investigated at room temperature with electric fields up to 2.5 kV mm−1 with the aim of developing anhydrous ER suspensions applicable over a wide temperature range. Anhydrous ER suspensions dispersing cellulose particles which were treated by phosphoric ester reaction of 2 mol phosphoric acid and 4 mol urea were measured; not only were the electrical characteristics such as dielectric constant, current density and electrical conductivity studied but also the rheological properties on the electric fields and volume fraction of dispersing particles. Anhydrous ER suspensions dispersing phosphoric ester cellulose particles showed very low current density, conductivity and very high performance of ER effect (τA/ τ0 ≈ 1030) on the 2.5 kV mm−1 electric field, and the yield stress (τy) had a quadratic dependence on electric fields. © 2000 Society of Chemical Industry

The mechanism of action and performance of dialkyl 3,5-di-t-butyl 4-hydroxy benzyl phosphonates as a new antiwear additive

Abstract A study of the effect of a series of additives in lubricating oil was carried out using a four-ball wear test machine. The ethyl derivative, diethyl 3,5-di- t -butyl 4-hydroxy benzyl phosphonate (DEP), of a series of newly synthesized derivatives showed excellent antiwear performance compared with zinc dialkyl dithio phosphate and TCP under high temperature and speed conditions. An approach based on the thermally activated wear theory has been applied. To investigate the hydrogen-scavenging role of the DEP additive, a thermal degradation test was conducted and 1 H nuclear magnetic resonance spectroscopy was used to identify the substitution reaction of hydrogen free radicals in the heated oil. On the basis of the experimental results, a new concept of antiwear mechanism was established and tested. The new additive showed the function of hydrogen and free-radical scavenging and was found to be a highly effective antiwear additive.

The Effect of Activation Time of Phosphoric Ester Cellulose Particles on the Electrorheological Properties of Anhydrous ER Fluids

The electrorheological (ER) behavior of suspensions in silicone oil of phosphoric ester cellulose powder (average particle size: 17.77 μm) was investigated at room temperature with electric fields up to 2.5 kV/mm. For development of anhydrous ER fluids using at wide temperature range, it was sought the effect of activation time of phosphoric ester cellulose particles on the ER activities. Anhydrous ER fluids based on the phosphoric ester cellulose particles made from cellulose particles treating in chemical solution of 2M phosphoric acid and 4M urea and heating at 150°C were measured. After activating the phosphoric ester cellulose anhydrous ER fluids at 120°C, not only analysis of dispersing cellulose particles but also electroroheological characteristics of ER fluids such as dielectric constant, current density, electrical conductivity and rheological properties were studied. Activation time had a large influence to ER properties of anhydrous ER fluids based on phosphoric ester cellulose. As the activation time went by, particle size and number of dispersing particles, electrical properties, dynamic yield stress on electric field, initial apparent viscosity (η0) and electrorheological effect (τA/τ0) of phosphoric ester cellulose ER fluids increased with increasing activation time at 120°C till activation time over 5 hours.